Conventionally, thick fiber structures have been widely used as sound-absorbing materials or heat-insulating materials for vehicles, houses, expressways, and the like. For example, sound-absorbing materials and heat-insulating materials containing various kinds of fibers, such as glass wool, urethane foam, and polyester fibers, are known. Characteristics required for such structures include sound-absorbing properties, heat-insulating properties, lightweight properties, and the like. In particular, as well as excellent heat-insulating properties, with respect to sound-absorbing properties, excellent sound-absorbing characteristics over a wide range from a low frequency to a high frequency are required.
The most common technique to satisfy the required characteristics, such as sound-absorbing properties and heat-insulating properties, is a method that simply reduces the fiber diameter or increases the areal weight. However, by simply reducing the fiber diameter, although certain frequencies can be dealt with, it has been difficult to deal with a wide range of frequencies. In addition, the increase of the areal weight has problems in that the lightweight properties are impaired, or the density becomes too high, resulting in low air permeability, whereby sound is reflected, and the sound-absorbing properties are rather reduced.
Thus, for example, various techniques of combining a plurality of fiber structures have been proposed. This is the idea that a plurality of fiber structures are laminated, for example, with the constituent fiber fineness or the like being varied among fiber structures, thereby satisfying required properties. For example, Patent Document 1 proposes a sound-absorbing structure obtained by laminating a nonwoven fabric having an average constituent fiber fineness of 0.1 to 2 dtex and a fiber structure having an average fineness of 0.5 to 10 dtex. In addition, Patent Document 2 proposes a lightweight sound-absorbing material obtained by integrally laminating a melt-blown nonwoven fabric having a fiber diameter of 6 μm or less and a short-fiber nonwoven fabric having a fiber diameter of 7 to 40 μm. Patent Document 3 proposes a sound-absorbing material obtained by laminating a melt-blown nonwoven fabric composed of fine fibers and a spun-bonded nonwoven fabric having a single-fiber fineness of 1 to 11 dtex.
However, there have been problems in that it is difficult to ensure adhesion during lamination, or a processing step is indispensable, leading to increased cost. In addition, because only a few kinds of fibers are combined, it has been difficult to sufficiently deal with a wide range of frequencies. The shape of a structure has been getting complicated for the mounting of various electronic components, etc., which is particularly notable in vehicle applications, and thus a structure having excellent shaping properties has been desired. However, in such a laminated structure, peeling, wrinkling, or the like occurs at the lamination interface, and it has been difficult to form various shapes with high quality.
In addition, as a method for the industrial mass production of thin fibers, melt-blown nonwovens fabrics and spun-bonded nonwoven fabrics as mentioned above are known. However, there have been problems in that such a method is a special production method in which a nonwoven fabric is directly formed on a net or the like, and also, with respect to the obtained nonwoven fabric, only a relatively thin nonwoven fabric can be obtained. That is, the problems in the laminating/processing step mentioned above have not yet been solved.
Thus, generally, heavy, thick fiber structures have been widely used to satisfy the sound-absorbing or heat-insulating characteristics. In particular, with respect to the ensuring of heat-insulating properties which are significantly affected by the thickness of the nonwoven fabric, a thick nonwoven fabric structure composed of fibers having a relatively high fineness has still been the mainstream.
The development of a fiber structure that sufficiently satisfies various characteristics, such as sound-absorbing properties, heat-insulating properties, and lightweight properties, and is easy to produce has been awaited.
(Patent Document 1) JP-A-2004-145180
(Patent Document 2) JP-A-2002-161464
(Patent Document 3) JP-A-2002-69824